Ammonia Pollution: Validation of Space-Based Emission Profiles

This week, I present to you a “News and Views” type science article that I had to write for a class last quarter! Depending on the popularity of this post (as well as my own time constraints) I would like to start writing more of these types of blog posts, particularly for areas more closely related to my specific research goals. Let me know if this is something you’re interested in!

Nitrogen is an essential nutrient that plants need to grow, but in the environment it mostly takes the form of unreactive N2 gas. It is only through the invention of the Haber-Bosch processes that N2 can be transformed into large quantities of reactive nitrogen species that can be utilized to increase crop production to levels that can sustain our world’s population. Of these manufactured forms of nitrogen, ammonia (NH3) dominates, accounting for 55% of man-made emissions with its main source being agricultural operations1. Due to our need to increase food production, ammonia emissions have more than doubled since preindustrial times and are predicted to continue increasing in the future2.

The addition of ammonia fertilizer, while essential for food production, has lead to a slew of unforeseen environmental effects. Following application, a substantial fraction of the NH3 evaporates from farms. After emission, the NH3 is redeposited into nearby terrestrial and aquatic environments. The former can result in soil acidification and the loss of species diversity, while the latter is affiliated with eutrophication and algae blooms that harm the local ecosystems3,4. Additionally, as NH3 is both highly reactive and the dominant basic gas in the atmosphere, it readily reacts with acidic species (e.g. sulfuric acid), neutralizing to form particulate matter that has been linked with reductions of air quality, increases in lung disease and impacts on global climate4.

Despite the well-documented environmental impacts, uncertainties as large as 50% are still present in the ammonia budget5. Until recently there was an insufficient monitoring of ammonia such that the spatial and temporal variability caused by its tight link to agricultural practices could not be quantified. Though some developed countries have created NH3 monitoring stations, in situ measurements are challenging, uncertain and not available in many regions, making it difficult to assess the sources, variability and eventual impacts of this critical nitrogen species3.

Recently remote sensing satellites have shown promise in closing this observational gap for ammonia. Since 2006, daily distributions of global ammonia concentrations have been obtained from the NASA Tropospheric Emissions Spectrometer (TES) and the Infrared Atmospheric Sounding Interferometer (IASI). During this time period, TES and IASI have provided insight on ammonia emissions on both global and regional scales, while observing at frequencies that enable the seasonal patterns associated with agriculture and seasonal biomass burning to be diagnosed6.

There remain inherent challenges in NH3 characterization. Ammonia is concentrated near the surface at concentrations of less than 10 parts per billion4. The combination of these two facts presents problems in observation, as it is difficult to see such compounds from space with high sensitivity6. Given these challenges, there is a critical need to validate these space-based measurements to make sure that they are accurate. Despite the increasingly common use of satellite NH3 observations, there have been very few attempts to check the accuracy of these data sources. Though one attempt was made using local monitoring networks, limits in the spatial and temporal representation the monitoring network precluded a quantitative comparison6,7.

Recently, Sun et al. validated the NASA TES NH3 observations using a combination of aircraft and vehicle-based in situ measurements. Using a high-frequency sampling vehicle and the NASA P-3B aircraft, NH3 measurements were made directly under the satellite transect within 1.5 hours of its overpass time. This method allowed for direct comparison between in situ and space-based observations at the surface and within the atmospheric layer.

This survey was performed in the San Joaquin Valley of California, an area known for having high, but variable ammonia emissions due to livestock and fertilizer use. At three separate transects over the area, retrievals from the satellite instrument were compared with aircraft data, mobile observations and a combination of the two. Overall the NH3 column from both space and in situ measurements agrees within 2%, which is less than both the satellite retrieval error and the instrument uncertainties. Compared to just the aircraft data, all values were within or close to the estimated errors. Lastly, data from the TES overpass was compared to surface mobile measurements. As expected, both of these sources of data showed elevated ammonia values downwind of dairies. This suggests that TES accurately captures the surface variability at a small, regional scales.

Satellite observations of NH3 are a valuable tool for diagnosing the ammonia budget. Sun et al. showed that the TES ammonia observations agree with in situ observations within error and captures surface variability under the conditions presented during this survey. Further validation under other, less optimal conditions is required to fully determine the accuracy of the TES ammonia observations. Additionally, the study design of Sun et al. is applicable to other satellite trace gas observations such as those performed by IASI and more recent NH3 monitoring satellites that have come online. Together, a combination of ground- and space-based observations will allow a better understanding about the evolution of ammonia from production through deposition into nearby ecosystems, allowing predictions of how global concentrations will change as ammonia continues to increase in the future.